1. Field of the Invention
The present invention relates to a structure of a contact chain for semiconductor product testing. In particular, the present invention relates to a contact chain that is capable of testing and failure analysis.
2. Description of the Related Art
In order to monitor the quality of semiconductor chips, several testing devices are fabricated on scribe lines between the semiconductor chips. For example, a PMOS or a NMOS on a scribe line should have similar electrical attributes with the PMOS or the NMOS in the semiconductor chips nearby. Thus, by testing or measuring the electrical attributes of the testing devices, one can obtain the device performance in the semiconductor chips. Among these testing devices, there is a specific test device referred to as a contact chain, serving to obtain an average resistance of the contacts in the semiconductor chips.
Contacts can be classified into at least 3 categories, CG, CS-P and CS-N. CG denotes the contact from the first metal layer to a gate layer or electrode; CS-P denotes the contact from the first metal layer to a P-type substrate; and CS-N denotes the contact from the first metal layer to an N-type substrate. Please refer to FIGS. 1A and 1B. FIG. 1A illustrates the layout of a conventional contact chain. FIG. 1B illustrates a cross-section of the contact chain in FIG. 1A, when it is implemented by contacts of CS-P. By examining the patterns of the P+ doped layers 12, contact holes 14 and the first metal strips 16, a plurality of contacts connected in series are easily observed. An average contact resistance of one contact of CS-P can be obtained by dividing the total resistance measured between Pad1 and Pad2 by the total number of the contacts in series. In other words, the average contact resistance of one contact of CS-P, CS-N or CG can be obtained from a contact chain of a respective kind of contact.
When the average contact resistance obtained by probing and measuring doesn""t fall into the allowable range, one or more contacts in the contact chain have problems. This induces an action called failure analysis to find out the root causes, so problems in the manufacturing process may be found and rectified.
One of the conventional tools for failure analysis is a focused ion beam (FIB), which functions like the well-known scanning electric microscope (SEM). By scanning an object with a positive-charged ion beam, the microscopic structure of the object can be observed. Two more functions of FIB are well known cutting an observed object by ion bombardment to achieve a cross-section view, and forming a connection route on its surface by depositing metal ions for circuit repairing.
Before analyzing the root cause of a failure contact chain by FIB, it must be polished to remove the metal strips, exposing the underlying dielectric layer.
FIGS. 2A and 2B show how the ion beam interacts with a normal, well-formed contact of CS-P and an abnormal, unfilled contact of CS-P, respectively. A contact chain always consists of hundreds of contacts. Therefore, without the exact location of an abnormal contact, failure analysis will be almost impossible. As shown in FIG. 2A, when the ion beam is scanning a normal contact of CS-P, positive charges can flow, through the conductive material inside the contact hole 14 and the forward-biased PN junction between the P+ doped layer and the N-type well 10, to the grounded N-type well 10. In other words, a normal contact of CS-P can discharge the charges carried by the impacted ions. When the ion beam scans an abnormal, unfilled contact of CS-P, as shown in FIG. 2B, positive charges are accumulated since the conduct material doesn""t fill the contact hole 14 enough to provide a conductive path, thus the positive charges arriving early repel the positive charges arriving late. The image formation theory of FIB is based upon the amount of secondary electrons induced by positive ions bombarding on the observed point of the object. In other words, the different interactions on surfaces cause the different gray levels on the monitor. If positive charges are accumulated, which occurs in FIG. 2B, the positive charges arriving late are repelled by the positive charges arriving early, such that secondary electron can""t be further generated. What occurs in the abnormal contact in FIG. 2B comes out of a gray level darker than that in the normal contact in FIG. 2A. Therefore, the normal and the abnormal contacts of CS-P are easily recognized by checking the gray levels shown on the monitor of an FIB tool.
However, FIB can""t be used to recognize normal contacts of CS-N and abnormal contacts of CS-N. FIGS. 2C and 2D show how the ion beam interacts with a normal, well-formed contact of CS-N and an abnormal, unfilled contact of CS-N, respectively. In FIG. 2C, the PN junction formed between N+ doped layer 20 and P-type well 18 is reverse-biased when the positive ion beam is scanning the contact when the P-type well 18 is grounded, thereby blocking the positive charges from discharging. Nor are the positive charges in FIG. 2D discharged since the conductive material in contact hole doesn""t occupy the contact hole sufficiently to form a conductive path. Therefore, all the contacts of CS-N, either normal or abnormal, accumulate positive charges in their contact hole and have similar gray levels displayed on the monitor of a FIB tool, so that they are difficult to differentiate from each other.
The main object of this invention is to provide a novel structure for a contact chain capable of recognizing the normal contacts of CS-N and the abnormal contacts of CS-N during FIB failure analysis.
Another object of this invention is to provide an analyzing method for recognizing the root cause of the failure contact chain of the present invention.
To achieve the mentioned objects, the present invention provides a structure of a contact chain comprising a substrate of a first conductive type, a dielectric layer on the substrate, a plurality of contact structures and two probe pads. The contact structures are connected in series and have two ends. Each contact structure comprises a contact hole in the dielectric layer and conductive material in the contact hole, for serving to electrically contact with a first doped layer of a second conductive type. The first doped region is formed on the substrate. Two probe pads are coupled to the two ends, respectively. The contact chain further comprises a means for selectively coupling the first doped layer to the substrate. When the substrate is not coupled to the first doped layer, the total contact resistance can be measured by probing the probe pads.
The present invention further provides a method for measuring the total resistance of a contact chain. The contact chain has a plurality of contact structures connected in series and two ends. Each contact structure comprises a contact hole in a dielectric layer and conductive material in the contact hole and serves to electrically contact with a first doped layer of a second conductive type. The first doped region is formed on a substrate of a first conductive type. Two probe pads are coupled to the two ends, respectively. A means for selectively coupling the first doped layer to the substrate is provided. The first doped layer is disconnected from the substrate. The contact chain is powered through the two probe pads. A voltage value is measured across the two probe pads and a current value is measured through one of the two probe pads to ascertain the total resistance.
The present invention further provides a debugging method for determining a failed contact structure among a plurality of contact structures. The contact structures are located on a substrate of a first conductive type. A dielectric layer is on the substrate. Each contact structure comprises a contact hole in the dielectric layer and conductive material in the contact hole, and serves to electrically contact with a first doped layer of a second conductive type. The first doped layer selectively coupled to the substrate is formed on the substrate. The steps of the debugging method follow. First, the substrate is coupled to the ground, and the first doped layer is selectively coupled to the substrate. A beam of charge carriers scans the contact structures to obtain first surface responses corresponding to the contact structures. A first specific contact structure is retrieved as the failure contact structure whose first response is unfit for a predetermined requirement.
With the aid of the means for selectively coupling the first doped layer to the substrate, it is very easy to find out a peculiar contact structure among the contact structures of the contact chain. Furthermore, more root causes of the failure contact chain can be determined by analyzing the contact chain of the present invention.